1. Field of the Invention
This invention relates to single package epoxy resin systems, which can be caused to harden into a cured state upon application of heat only. More specifically, this invention relates to a "hardening monomer" for epoxy resins which is insoluble in liquid resin polymers at low temperatures.
2. Description of the Prior Art
Curable epoxy resin systems have long been employed for a variety of important utilities, particularly as adhesives, and resins for structural elements. Conventionally, use of an epoxy required the application of the uncured epoxy polymer to the surface or area wherein the cured resin is to be formed, and thereafter addition of a "hardening monomer" which, upon the application of heat, caused the polymer to undergo crosslinking and curing, into a "hardened" state. Unfortunately, the application of such resin systems was frequently performed by those without particular skill in the art, and the extensive handling of the "two package" system of a polymer and a monomer resulted in messy and inferior products.
Recently, industry efforts have been focused on the provision of "one package" epoxy resin systems, wherein the hardener is dispersed within the epoxy polymer, but inactive in the stored state. The system may be applied to the article to be provided with the epoxy resin, and, upon the application of heat, the epoxy resin hardens. However, problems have continued to plague the provision and use of such "one package" systems.
One particular problem encountered has been the extremely short shelf life of the one package systems. Over time, the hardening monomer tends to react with the polymer, even in the absence of the application of heat. Thus, in a matter of a few days, or weeks, the system may become unusable, due to premature curing. European patent application No. 0024119, published Feb. 23, 1981 addresses a one package system employing a hardener identified, in a preferred form, as a succinic acid salt of imidazole modified by phenyl isocyanate. However, when combined with an epoxy resin of polyglycidyl ether, the system was found to have only a 2-3 day shelf life.
Another problem frequently encountered is the high heat necessary for curing, or the long duration of exposure to elevated heat necessary for curing the one package system. Dicyandiamide is widely used as a curative for epoxy resins, particularly in "one package" systems, as the storage stability of such a system at room temperatures of 70.degree.-78.degree. F. is excellent. However, the temperatures required to bring the curing reaction to completion (in excess of 350.degree. F.) are too high for many of the preferred applications of epoxy resins, such as their use in bonding adhesives, prepregs, laminates and coatings. Further, in many cases, these high temperatures must be maintained for prolonged periods. The curing of the epoxy resin system may be accelerated by addition of various amine salts. However, the presence of the additives severely, adversely effects the strength of the cured product, and storage durability of the one package system. Further, many additives have only a slight accelerating effect.
In an effort to overcome the above-described difficulties, U.S. Pat. No. 4,335,228 describes as curing catalysts isocyanate blocked imidazole or imidazoline compounds. These catalysts, and the one package systems employing them, are described as having outstanding shelf life, lower curing temperatures, and faster cure times. However, the "lower curing temperatures" of those systems are still on the order of 250.degree. F. to 400.degree. F. (column 2, line 58), which may be too high for many important applications. Of even greater importance is the limitation of the curing catalyst of U.S. Pat. No. 4,335,228 to use with epoxy resins which are solid at room temperature, and preferably have melting points of 60.degree. C. or more. Column 3, lines 12-16. Many important epoxy resins, including some of the diglycidyl ethers referred to above, are viscous liquids at room temperatures, and not suitable for use with the curing agents of the referenced patent.
Accordingly, a need continues to exist for a curing agent for epoxy resins which can be incorporated in a one package system which is suitable for use in liquid resins, has an extended shelf life and fairly quick cure times at relatively low temperatures.